Out of resin materials, an ethylene polymer and an ethylene-based polymer such as copolymer of an ethylene and an α-olefin are excellent in the physical properties or various properties such as moldability and have a superiority in view of profitability, environmental compliance and the like, and these materials have been heretofore very widely employed for general purposes and are being used as an important industrial material. However, the ethylene-based polymer has no polar group and its application to the field requiring physical properties such as adhesion to another material, printing suitability or compatibility, for example, with a filler is limited. In the application where physical properties such as adhesion to another material, printing suitability or compatibility, for example, with a filler are required, a copolymer of an ethylene and a polar group-containing vinyl monomer, produced by a high-pressure radical polymerization process, has been used by itself or as a composition with another resin. However, the polar group-containing ethylene-based polymer produced by high-pressure radical polymerization can be only a low-modulus material and is poor also in the mechanical properties, and its application particularly to a field requiring high strength is limited even when used as a composition with another resin as well as when used by itself.
Since the 1990s, polar group-containing comonomer copolymerization using a late transition metal complex catalyst has been aggressively studied, and there are known, for example, an (α-diimine)palladium complex reported by Brookhart et al., a (salicylamidinato)nickel catalyst reported by Grubbs et al., and a (phosphanylphenolato)nickel catalyst called a SHOP catalyst. In use of such a catalyst, the polymerization temperature is set to be relatively low so as to suppress a frequent occurrence of chain transfer, and the productivity of the copolymer as well as its molecular weight are generally low (see, for example, Non-Patent Document 1).
In 2002, Pugh et al. have reported that when a phosphine sulfonate ligand is combined with a palladium compound and used as a catalyst component, copolymerization even at a high temperature (80° C.) can be performed (see, Patent Document 1 and Non-Patent Document 2), and this technique enables realizing high productivity and moreover, ensures a relatively high content of a (meth)acrylic ester as a comonomer. However, the molecular weight (Mw) of the copolymer obtained has an upper limit of about tens of thousands and therefore, industrial application of this copolymer is also limited.
The phosphine sulfonate ligand above is estimated to be a chelating or potentially chelating ligand and has been reported, for example, to become a chelated metal complex by complexing with palladium (Non-Patent Document 3). Also, it has been reported that a phosphine carboxylate ligand having a —CO2H group becomes a chelated metal complex by complexing with nickel (Non-Patent Document 4).
Nozaki et al. have reported that a (phosphine-sulfonato)palladium (methyl)lutidine complex is isolated as a catalytically active component and this is useful as a catalyst (see, Patent Document 2 and Non-Patent Document 3). In this case, the catalytic activity is greatly enhanced, but the molecular weight still remains low.
Ethylene polymerization and ethylene/1-hexene copolymerization each using the isolated (phosphine sulfonato)palladium (methyl)lutidine complex have been reported by Jordan et al. (Non-Patent Document 6). The report says that this catalyst does not absorb 1-hexene in the case of polymerization under an ethylene pressure (3 MPa) but copolymerizes a slight amount of hexene in the case of a low ethylene pressure (0.5 MPa).
Goodall et al. have developed a phosphine sulfonate ligand having a biphenyl structure by improving the phosphine sulfonate ligand (see, for example, Patent Documents 3 to 8 and Non-Patent Document 5). It is disclosed that by using this ligand as a catalyst for the copolymerization of an ethylene and an acrylic ester, a copolymer having a molecular weight (Mw) of 100,000 or more can be produced. However, according to the evaluation by the present inventors, it has been found that the comonomer content disadvantageously decreases.
Accordingly, in the field of copolymerization of an ethylene and a vinyl acetate as a polar group-containing vinyl monomer or a ((meth)acrylic acid)-based olefin, development of a polymerization catalyst capable of satisfying both high copolymerizability and high molecular weight (Mw) is being demanded.
On the other hand, a ternary copolymer of an ethylene, an α-olefin and a polar group-containing monomer is also known and, for example, an ethylene•1-octene•ethyl acrylate ternary copolymer having an ethyl acrylate content of 12.1 to 35.5 mol % and being produced using a specific chromium-based catalyst is disclosed in Patent Document 9. However, this polymer is yet insufficient in view of improving the balance between the mechanical properties and the adhesion and moreover, it still leaves problems such as many sticky components, generation of die lip build up at the molding, and film blocking. Also, Patent Document 10 discloses an ethylene•propylene•methyl acrylate ternary copolymer having a propylene content of 13.5 to 18.5 mol % and a methyl acrylate content of 8 to 27.2 mol % and being produced using a specific vanadium-based catalyst, but the studies by the present inventors have revealed that the strength of the polymer obtained is below the level expected of a polyethylene as a material. In this way, compared with an ethylene-based (co)polymer containing no polar group, great reduction in the mechanical properties of an ethylene-based copolymer containing a polar group is inevitable.